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Time capsule galaxy can help probe conditions after the big bang

Kitt Peak Observatory tests the metal of the “little lion”

Babak Tafreshi/National Geographic Creative

By Jacob Aron

Astronomers have discovered a galaxy that seems to have stayed relatively unchanged since forming shortly after the big bang. Studying this time capsule will help us understand cosmic conditions in the deep past, and learn how the earliest galaxies formed stars.

Shortly after the big bang, the only elements in the universe were hydrogen and helium, with a few traces of other light elements. Heavier elements, which astronomers refer to as “metals”, were only created after the hydrogen and helium formed into stars, which forged new elements through nuclear fusion.

Because of this, astronomers measure the abundance of metals within stars and galaxies to track their evolution. Now Alec Hirschauer of Indiana University in Bloomington and his colleagues have measured a small galaxy called AGC 198691, which is around 30 million light years from Earth and has the lowest metallicity ever seen.

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“This galaxy is a close analogue to what we expect galaxies were like shortly after they formed, before they have had the time to chemically enrich to the levels we see in local, nearby systems,” says Hirschauer. “It is a nearby laboratory we can use to approximate the conditions soon after the big bang.”

NASA

The galaxy is near the constellation of Leo Minor, so the team have nicknamed it the Leoncino dwarf. To analyse the light from Leoncino, the team used telescopes in Arizona at the Kitt Peak National Observatory in Tucson, and the MMT Observatory in Mount Hopkins, and found that that the galaxy’s ratio of oxygen to hydrogen – a proxy for overall metallicity – is the lowest ever seen, just 2 per cent of our sun’s metallicity.

Leoncino hosts bright blue stars, which could only have formed relatively recently. That means the galaxy probably forms stars very slowly, and is only starting to use up material hanging around since the big bang.

Astronomers don’t yet understand why some galaxies form stars slower than others, though it seems to be related to their mass. “AGC 198691 is a small galaxy, and so has been inefficient at converting gas into stars over the length of time that it has existed,” says Hirschauer.

Studying Leoncino further will help deepen our understanding of the differences between galaxies, and also give us a look back at how galaxies behaved in the early universe. “By studying how the stars of AGC 198691 are forming, we have a glimpse into what very early galaxies were doing,” says Hirschauer.